It is known that the force necessary to close a valve is not constant and changes based on the travel of the moving member, in the manner illustrated in FIG. 2. During the long approach phase of the moving member, i.e., before the arrival on the sealing seat of the valve, the necessary force is relatively constant and low and only increases considerably in the short tightening phase up until closing of the valve, the duration of which is generally shorter than 10% of the duration of the total travel. Yet in manually controlled hand wheel servomotor systems, which are known, the manual control mechanism of the valve is designed solely as a function of the short high-force phase without taking into account the much longer approach phase.
The invention aims to offset this drawback.